Die, especially for extruding green ceramic foils

ABSTRACT

A die for producing flat sections, in particular a flat-sheet die for extrusion of ceramic green films, has a slot-shaped die outlet which opens in an outlet orifice. The height of the slot-shaped die outlet narrows beginning in the immediate vicinity of the outlet orifice and continuing to the outlet orifice. The die outlet is preferably formed by a first lip and a second lip situated opposite the first, having a constant first distance in at least some areas outside the immediate vicinity of the outlet orifice, and having at the outlet orifice a second smaller distance in comparison with the first distance, the first distance between the lips decreasing linearly to the second distance in the area of the immediate vicinity of the outlet orifice.

FIELD OF THE INVENTION

[0001] The present invention relates to an extrusion die for producingflat sections, in particular a flat-sheet die for extrusion of ceramicgreen films.

BACKGROUND INFORMATION

[0002] An extrusion system having a flat-sheet die exhibiting a diedesign known from plastics technology is generally used for extrusion ofpolymer-bonded ceramic flat sections, i.e., films and green films. Asshown in FIG. 1, such dies have an inlet, a distributor channelincluding a throttle field and a die outlet, i.e., a die relief having avariable gap height, i.e., a flex-lip, using which the thickness of theextruded flat sections is adjustable.

[0003] However, extrusion of ceramic sections in particular differsfundamentally from extrusion of plastics in two regards. First, theviscosity (i.e., the intrinsic viscosity in the case of intrinsicallyviscous substances, in which the viscosity depends on the shear rate) ofextruded ceramic pastes is greater than that of plastics, andfurthermore, a ceramic paste generally has a flow limit. In addition, inextrusion of flat ceramic sections, the fact that subsequent processingsteps such as removal of binders or sintering may be very sensitive intheir response to stresses frozen into the extruded flat sections shouldbe taken into account.

[0004]FIG. 2 illustrates the flow velocity profile of a ceramic pasteextruded through a conventional flat-sheet die in the outlet of the diefor various viscosities of the extruded ceramic pastes, showing that therate of flow is zero at the edges of the die and it is maximum at thecenter. The viscosity or intrinsic viscosity of the extruded ceramicpaste is characterized by the flow exponent, which is defined by:

{dot over (γ)}=Φτ^(m)

[0005] where {dot over (γ)} is the gradient of the flow velocity, i.e.,the shear rate in the extruded paste, Φ is the fluidity and τ is theshear stress. Accordingly, the following equation holds for theviscosity, i.e., intrinsic viscosity η of the ceramic paste:$\eta = {\Phi^{- \frac{1}{m}}{{\overset{.}{\gamma}}^{({\frac{1}{m} - 1})}.}}$

[0006]FIG. 2 shows that shear rate {dot over (γ)} is zero at the centerpoint of the die outlet and is maximum at the edge. The shear rate atthe edge of the die is {dot over (γ)}_(w). Shear rate {dot over (γ)}_(w)at the wall is calculated as follows at a given volume throughput {dotover (V)}:${\overset{.}{\gamma}}_{w} = {2( {m + 2} )\frac{\overset{.}{V}}{H^{2}B}}$

[0007] where m again denotes the flow exponent of the extruded paste andis thus a measure of the intrinsic viscosity of this paste, and Hdenotes the height of the flat-sheet die at the site in question and Bis its width.

[0008]FIG. 2 shows in particular that, when there is a flow limit, i.e.,a high flow exponent m, shearing of the extruded material occurs almostexclusively in the edge area of the die. In the case of extrudedpolymer-bonded ceramic pastes, this shearing produces an orientation ofthe binder molecules added to these pastes, which may result inconsiderable after-shrinkage of the extruded flat sections in the caseof a downstream add-on processing used on the extruded flat sections,e.g., imprinting by screen printing or lamination. Since the degree ofmolecular orientation established in the extruded flat sections is notconstant over the thickness of this film due to the differences in shearrates, such a post-shrinkage, i.e., relaxation which is associated witha macroscopic change in shape, cannot usually take place completely.Thus, in the past there have always been residual internal stresses,which are manifested as unwanted changes in geometry (shrinkage) insubsequent printing steps or lamination steps. In addition, delaminationsimilar to puff pastry also frequently occurs near the film surfacesduring a subsequent sintering.

SUMMARY OF THE INVENTION

[0009] An object of the present invention was to provide an extrusiondie for producing flat sections which would avoid the aforementioneddisadvantages and in particular would ensure a more homogeneous shearrate and a more homogeneous gradient of the molecular orientation overthe thickness of the extruded flat sections.

[0010] The extrusion die according to the present invention forproducing flat sections has the advantage over the related art that theflat sections produced with this die have a substantially reducedanisotropic shrinkage in a subsequent annealing in comparison with therelated art. Furthermore, there is less post-shrinkage during subsequentimprinting steps or lamination. In addition, it is advantageous that dueto the embodiment of the extrusion die according to the presentinvention, no flaky delamination is observable in sintering the flatsections produced in this way and no “chatter marks” occur due tostick-slip behavior, i.e., an uneven scaly surface of the extrudatecaused by discontinuities in the flow curve in extrusion.

[0011] In addition, it is advantageous that apart from the modificationof the extrusion die outlet according to the present invention,flat-sheet dies that are otherwise known from the related art may alsobe used for extrusion of ceramic green films or plastic films. Due tothe fact that only the area of the die outlet in the immediate vicinityof the outlet orifice of the die is altered due to a particulargeometric design, a die according to the present invention is muchsimpler to manufacture and may be integrated into existing or knownextrusion systems. The cost of manufacturing the dies according to thepresent invention is therefore comparable to that of the known dies, andno significant investments in new extrusion systems are necessary.

[0012] It is particularly advantageous if the die outlet is formed by afirst lip and a second lip situated opposite the first, having aconstant first distance in at least some areas outside the immediatevicinity of the slot-shaped outlet orifice, and then having a secondsmaller distance in comparison with the first distance at the outletorifice, the first distance in the area of the immediate vicinity of theoutlet orifice narrowing linearly to the second distance. It isparticularly advantageous if this linear narrowing from the firstdistance to the second distance takes place with an angle of inclinationwhich is adapted to the flow exponent of the extruded ceramic paste.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 shows a flat-sheet extrusion die known from the related artfor extrusion of ceramic green films.

[0014]FIG. 2 shows the flow velocity profile of extruded pastes in thedie outlet of the extrusion die according to FIG. 1 as a function offlow exponent m.

[0015]FIG. 3 shows a die relief according to the present invention of aflat-sheet die according to FIG. 1 in a sectional view.

[0016]FIG. 4 illustrates the definition of angle of inclination a.

DETAILED DESCRIPTION

[0017] The present invention first relates to flat-sheet extrusion die10 for extrusion of ceramic green films such as those known in principlefrom the related art. In particular, such a known flat-sheet die 10according to FIG. 1 has an inlet for supplying the fluid substancecoming out of an outlet orifice 13 and a downstream distributor channelhaving a throttle field opening into a die outlet 20, i.e., a dierelief. A first lip 11 and a second lip 11′ situated opposite the firstare provided in this die outlet, initially spaced a distance of 1.5 mmapart, for example, then narrowing in front of outlet orifice 13 to asmaller second distance of 0.5 mm, for example, this second distancethen remaining constant after narrowing until outlet orifice 13. To thisextent, in the related art, according to FIG. 1, the distance of firstlip 11 from second lip 11′ is constant in the immediate vicinity ofoutlet orifice 13.

[0018]FIG. 2 shows the flow velocity profile of the ceramic paste in dieoutlet 20 in an area 12 which ends at outlet orifice 13, the flowvelocity being plotted on the x axis and the height on the z axis. Inparticular, the flow velocity is zero at the walls of lips 11, 11′.

[0019]FIG. 2 shows that the gradient of the flow velocity, i.e., theshear rate in the paste, is particularly high in the vicinity of thewalls, i.e., near the edges of die 10, i.e., lips 11, 11′. This effectis particularly pronounced in the case of pastes having a high flowexponent m, i.e., pastes having a flow limit.

[0020] In a modification of FIG. 1, FIG. 3 shows a die outlet 20according to the present invention of a flat-sheet die for extrusion ofceramic green films, in particular polymer-bonded green films, having afirst lip 11 and a second lip 11 ′ opening into a slot-shaped outletorifice 13. Die outlet 20 according to FIG. 3 differs from the one inFIG. 1 in that the height of slot-shaped die outlet 20 narrowscontinuously and linearly, beginning in an immediate vicinity 12′ ofslot-shaped outlet orifice 13 and continuing to slot-shaped outletorifice 13. To this extent, lips 11, 11 ′ are initially spaced aconstant first distance h₁ apart, this distance narrowing linearly tosecond, smaller, distance h₂ in immediate vicinity 12′ of outlet orifice13.

[0021] In this way, die outlet 20 according to FIG. 1 having a long,plane-parallel die relief in area 12 of the ultimate gap height isreplaced by a blade-like outlet of the die relief in a narrowed areawhich forms immediate vicinity 12′ of outlet orifice 13. Blade-shapednarrowed area 12′ directly in front of outlet orifice 13 thus results inthe extruded flat sections not being shaped to desired thickness h₂until directly in front of outlet orifice 13. Since the height of thedie outlet, i.e., first distance h₁, is greater further upstream fromthis narrowing 12′ than in area 12 according to FIG. 1, this results ina reduced maximum shear rate {dot over (γ)} and thus a reduced shearrate gradient.

[0022] Due to blade-shaped narrowed area 12′ in die outlet 20 accordingto FIG. 3, an elongation flow is superimposed on the shear flow there,likewise causing an orientation of polymer molecules or binder moleculescontained in the polymer-bonded ceramic paste, for example, in the xdirection; however, this elongation is homogeneous over the thickness ofthe film, i.e., homogeneous with respect to the z direction. Inparticular, the entire extruded paste then undergoes deformation, andregions which do not flow cannot form, as is the case with a shear flowdue to the flow limit. Therefore, due to the die geometry according tothe present invention, the gradient of the binder molecular orientationin the extruded flat sections is reduced.

[0023] A particularly advantageous embodiment of the present inventionprovides that the linear narrowing from first distance h₁ to seconddistance h₂ takes place in narrowing area 12′ having an angle ofinclination a for which it is at least approximately true that:

tan α=m+2

[0024] where m is again the flow exponent of the paste, e.g., theceramic polymer-bonded paste emerging through outlet orifice 13. Thedefinition of a is also obtained from FIG. 4.

[0025] The choice of angle α explained here yields the result that therate of elongation and the shear rate in die outlet 20 are of the sameorder of magnitude. In particular, this yields the results that the rateof elongation in blade-shaped narrowed area 12′ is equal to the maximumshear rate, i.e., the shear rate at the wall.

[0026] It should also be pointed out that first distance h₁ between thetwo lips 11, 11 ′ is between 250 μm and 40 mm, in particular 250 μm to 2mm, and second distance h₂ at outlet orifice 13 is between 70 μm and 10mm, in particular 70 μm to 1 mm. To this extent, the narrowing fromfirst h₁ to second distance h₂ is usually established at a distance of 1mm to 5 mm upstream from outlet orifice 13, depending on the settings ofthese values and the angle of inclination α. This range definesnarrowing area 12′ in the immediate vicinity of slot-shaped outletorifice 13. The width of flat-sheet die 10 may be up to 4 meters.

[0027] In conclusion, it should be emphasized that first distance h₁ andthus also second distance h₂ are adjustable variably in extrusion die 10in a known manner.

What is claimed is:
 1. A die for producing flat sections, in particulara flat-sheet die for extrusion of ceramic green films, having aslot-shaped die outlet which opens in an outlet orifice, wherein theheight of the slot-shaped die outlet (20) narrows beginning in theimmediate vicinity of the outlet orifice (13) and continuing to theoutlet orifice (13).
 2. The extrusion die as recited in claim 1, whereinthe narrowing is a continuous narrowing.
 3. The extrusion die as recitedin claim 1 or 2, wherein the die outlet (20) is formed by a first lip(11) and a second lip (11′) situated opposite the first; outside theimmediate vicinity of the outlet orifice (13), the first and second lipsbeing spaced apart at a constant first distance (h₁), at least in someareas, and, at the outlet orifice (13), the first and second lips beingspaced apart at a second distance (h₂) that is less than the firstdistance (h₁); and, in the region in the immediate vicinity of theoutlet orifice (13), the first distance (h₁) between the lips (11, 11′)decreasing linearly to the second distance (h₂).
 4. The extrusion die asrecited in claim 3, wherein the linear narrowing from the first distance(h₁) to the second distance (h₂) takes place with an angle ofinclination (α) for which the equation tan α=m+2 applies at leastapproximately, where m is the flow exponent of a fluid substancedischarged through the outlet orifice (13).
 5. The extrusion die asrecited in claim 3 or 4, wherein the first distance (h₁) is between 250μm and 2000 μm and the second distance (h₂) is between 70 μm and 1000μm.
 6. The extrusion die as recited in claim 1, wherein the narrowingbegins at a distance of 1 mm to 5 mm in front of the outlet orifice(13).
 7. The extrusion die as recited in one of the preceding claims,wherein the die outlet (20) is formed in the shape of a blade in theimmediate vicinity of the outlet orifice (13).
 8. The extrusion die asrecited in one of the preceding claims, wherein the first distance (h₁)and/or the second distance (h₂) is variably adjustable.
 9. The extrusiondie as recited in one of the preceding claims, wherein an inlet forsupplying the fluid substance discharged through outlet orifice (13) isprovided, as well as a downstream distributor channel that has athrottle field and opens into the die outlet (20).
 10. Use of anextrusion die as recited in one of the preceding claims, in an extrusionsystem for extrusion of polymer-bonded, ceramic flat sections or greenfilms.